US EPA

4848220 

Journal Article 

MoO3 as p-type dopant for Alq(3)-based p-i-n homojunction organic light-emitting diodes 

Kao, PoC; Chiu, CTzu 

2015 

1 

Organic Electronics
ISSN: 1566-1199 

26 

443-450 

10.1016/j.orgel.2015.08.018 

WOS:000361226900067 

Using high-work-function material MoO3 as a p-type dopant, efficient single-layer hybrid organic light-emitting diodes (OLEDs) with the p-i-n homojunction structure are investigated. When MoO3 and Cs2CO3 are doped into the conventional emitting/electron-transport material tris-(8-hydroxyquinoline) aluminum (Alq(3)), respectively, a significant increase in p-and n-type conductivities is observed compared to that of intrinsic Alq(3) thin films. With optimal doping, the hole and electron mobilities in Alq(3): MoO3 and Alq(3): Cs2CO3 films was estimated to be 9.76 x 10(-6) and 1.26 x 10(-4) cm(2)/V s, respectively, which is about one order of magnitude higher than that of the undoped device. The p-i-n OLEDs outperform undoped (i-i-i) and single-dopant (p-i-i and i-i-n) OLEDs; they have the lowest turn-on voltage (4.3 V at 1 cd/m(2)), highest maximum luminance (5860 cd/m(2) at 11.4 V), and highest luminous efficiency (2.53 cd/A at 100 mA/cm(2)). These values are better than those for bilayer heterojunction OLEDs using the same emitting layer. The increase in conductivity can be attributed to the charge transfer process between the Alq(3) host and the dopant. Due to the change of carrier concentration in the Alq(3) films, the Fermi level of Alq(3) is close to the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels upon p-and n-type doping, respectively, and the carrier injection efficiency can thus be enhanced because of the lower carrier injection barrier. The carriers move closer to the center energy levels of the HOMO or LUMO distributions, which increases the hopping rate for charge transport and results in an increase of charge carrier mobility. The electrons are the majority charge carriers, and both the holes and electrons can be dramatically injected in high numbers and then efficiently recombined in the p-i-n OLEDs. As a result, the improved conductivity characteristics as well as the appropriate energy levels of the doped layers result in improved electroluminescent performance of the p-i-n homojunction OLEDs. (C) 2015 Elsevier B.V. All rights reserved. 

Organic light-emitting diode; Single-layer; Homojunction; Doping